Method and apparatus for flaw detection by leakage fluxes and leakage flux sensor

Abstract

A leakage flux flaw detection apparatus in which an object material is magnetized and leakage fluxes derived from a flaw are detected to thereby search for a flaw of the object material in a predetermined flaw-detection direction. The apparatus has a pair of magnet poles for magnetizing the object. material in a direction different from the flaw-detection direction, another pair of magnet poles for magnetizing the object material in a direction different from the magnetization direction and flaw-detection direction of the first pair of magnet poles at a predetermined distance in the flaw-detection direction from the magnetization area of the two magnet poles, and two leakage sensors each of which is interposed between each of the two pairs of magnet poles. The leakage flux sensor which detects leakage fluxes from a flaw of the magnetized object material through two flux-sensing parts arranged in opposed relation to the object material and in predetermined spaced relation with each other along the flaw-detection direction includes two flux-sensing parts having the center distance d thereof not more than 4 mm, the length L of not less than 0.5 mm in the direction perpendicular to the flaw-detection direction, and the value d/L of not less than 1. The leakage flux sensor includes two concentrically-arranged flux-sensing parts having circular cross sections with different cross sectional areas.

I claim: 
     
       1. A method for flaw detection by leakage fluxes in which an object material is magnetized by a pair of magnet poles and leakage fluxes are detected by a sensor interposed between said pair of magnet poles while searching for a flaw of the object material in a predetermined flaw-detection direction, comprising the steps of: magnetizing the object material by a pair of magnet poles in a direction different from the flaw-detection direction;   magnetizing the object material by another pair of magnet poles in a direction different from the magnetization direction and flaw-detection direction in predetermined spaced relation from the magnetization area of said two magnet poles in the flaw-detection direction;   detecting the leakage fluxes from the object material by each of sensors interposed between each of the two pairs of magnet poles; and   identifying a flaw of the object material on the basis of the detection result of leakage fluxes wherein the step of identifying a flaw includes the steps of: calculating the output ratio between said two sensors;   determining the tilt angle of a flaw on the basis of a predetermined relationship between the output ratio and the flaw tilt angle on the one hand and the calculated output ratio on the other hand;   correcting the amplitude of the outputs of said two sensors on the basis of a predetermined relationship between the flaw tilt angle and the amplitude change rate of the sensor output signal on the one hand and the tilt angle of the target flaw determined on the other hand; and   calculating the depth of the flaw on the basis of the corrected amplitude of the outputs.     
     
     
       2. An apparatus for flaw detection by leakage fluxes in which an object material is magnetized by a pair of magnet poles and leakage fluxes are detected by a sensor interposed between said two magnet poles while searching for a flaw of the object material in a predetermined flaw-detection direction, comprising: a first pair of magnet poles positioned for magnetizing the object material in a direction different from the flaw-detection direction;   a second pair of magnet poles for magnetizing the object material in a direction different from the magnetization direction and flaw-detection direction in predetermined spaced relation from the magnetization area of said two magnet poles in the flaw-detection direction;   two sensors each of which is disposed between each of said first and second pairs of magnet poles for sensing magnetic flux;   means for calculating the output ratio between said two sensors;   means for determining the tilt angle of the target flaw on the basis of a predetermined relationship between the output ratio and the flaw tilt angle on the one hand and the calculated output ratio on the other hand;   means for correcting the amplitude of the output signals of said two sensors on the basis of a predetermined relationship between the flaw tilt angle and the amplitude change rate of the sensor output signal on the one hand and the tilt angle of the flaw determined on the other hand; and   means for calculating the depth of the flaw on the basis of the corrected amplitude of the output signals.   
     
     
       3. An apparatus for flaw detection by leakage fluxes in which an object material is magnetized by a pair of magnet poles and leakage fluxes are detected by a sensor interposed between said two magnet poles while searching for a flaw of the object material in a predetermined flaw-detection direction comprising: a first pair of magnet poles positioned for magnetizing the object material in a direction different from the flaw-detection direction;   a second pair of magnet poles for magnetizing the object material in a direction different from the magnetization direction and flaw-detection direction in predetermined spaced relation from the magnetization area of said two magnet poles in the flaw-detection direction;   two sensors each of which is disposed between each of said first and second pairs of magnet poles for sensing magnetic flux wherein said first and second pairs of magnet poles have changeable directions of magnetization and said two sensors have changeable detection areas;   means for calculating the output ratio between said two sensors;   means for determining the tilt angle of the target flaw on the basis of a predetermined relationship between the output ratio and the flaw tilt angle on the one hand and the calculated output ratio on the other hand;   means for correcting the amplitude of the output signals of said two sensors on the basis of a predetermined relationship between the flaw tilt angle and the amplitude change rate of the sensor output signal on the one hand and the tilt angle of the flaw determined on the other hand; and   means for calculating the depth of the flaw on the basis of the corrected amplitude of the output signals.